Driving a quadrpole mass spectrometer via an isolating stage

ABSTRACT

A power supply for a quadrupole mass spectrometer which operates using an RF signal. The RF signal is controllable via a feedback loop. The feedback loop is from the output, through a comparator, and compared to a digital signal. An air core transformer is used to minimize the weight. The air core transformer is driven via two out of phase sawtooth signals which drive opposite ends of the transformer.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. application Ser. No.09/392,351, filed Sep. 8, 1999 (allowed), which claims the benefit ofU.S. Provisional Application No. 60/099,630, filed on Sep. 8, 1998.

STATEMENT OF THE FEDERALLY FUNDED RESEARCH

[0002] The invention described herein was made in the performance ofwork under a NASA contract, and is subject to the provisions of PublicLaw 96-517 (35 U.S.C. 202) in which the Contractor has elected to retaintitle.

BACKGROUND

[0003] Certain applications, including a quadrupole mass spectrometer,can require a specialized power supply.

[0004] A power supply for this purpose has specialized requirements. Itshould be a high frequency power supply that has a variable peak RFamplitude, but is frequency and voltage stable once set. It should alsobe fully floating. These power supplies should also be capable ofdriving a primarily capacitive load.

[0005] If the device will be operating unattended or in space, the powersupply should also be lightweight and efficient.

SUMMARY

[0006] The present disclosure teaches a stable, high amplitude, highfrequency radio frequency and direct current power supply system.According to one aspect, the system uses a clocked operation to turn onpower from a power supply.

[0007] A high dynamic range power supply is described that has an anoscillator assembly operating from a first power supply and producefirst and second out-of-phase, gradually increasing, signals, first andsecond transistors, coupled to receive said first and second signalsrespectively, and turned on by the signals to produce an oscillatingoutput. The first transistor produces a first part of the oscillatingoutput and the second transistor produces a second part of theoscillating output. A feedback loop has a detector sensing a level ofthe oscillating output and producing a signal indicative thereof. Asecond element compares that signal to a reference and produces an erroroutput indicative of the difference, said error output causing a changein said first and second drive signals. The first transistor isreferenced to a second power supply, having a different level than thefirst power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 shows a schematic of the system.

DETAILED DESCRIPTION

[0009] The system is shown in detail in FIG. 1. A clock 102 produces abasic high frequency output 104, here shown as a 20 megahertz clock. Itshould be understood that any other frequency could be used. A flip flopamplifier 106 divides the oscillating output 104 into two, out-of-phase10 megahertz signals 108 and 110. The in-phase 10 megahertz signal 108is taken as a baseline (zero) phase shift, while the out-of-phase 10 MHZsignal 110 is shifted by 180 degrees relative to signal 108.

[0010] The output signals 108 and 110 are provided into two analogous,but out-of-phase circuits.

[0011] The integrator/summing amplifier 120 is shown as an operationalamplifier with a capacitor C1 and resistor R4 in its feedback loop. Thiseffectively changes the square wave output 108 into a graduallyincreasing signal such as a sawtooth shape having a similar frequency tothe driving signal. The sawtooth frequency is applied to the gate ofMOSFET 130, and periodically turns on the MOSFET 130. When MOSFET 130 isturned on, it drives current from the power supply 140 to one end of theprimary 152 of an air core transformer 150. The return for the powersupply 140 is coupled to the center tap 154 of the air core transformer150. Use of a air core transformer can reduce the weight of the system.

[0012] MOSFET 130 begins conducting when the sawtooth level reaches thethreshold voltage (Vth) of the MOSFET 130. As the level of the sawtoothincreases at the gate of MOSFET 130, the conduction angle increases. AsMOSFET 130 turns on more completely, it conducts more current. Thephase-shifted signal 110 is analogously coupled through an amplifier 122to an analogous MOSFET 132. The two circuits operate similarly, but 180degrees out-of-phase. When MOSFET 130 is in its active phase, MOSFET 132is off. Conversely, when MOSFET 132 is in its active phase, the MOSFET130 is off. In this way, the primary 152 of transformer 150 is beingalternatively pushed and pulled from opposite directions by twoout-of-phase 10 MHZ signals. The output is therefore proportional to theamount of pushing and pulling that occurs.

[0013] The secondary 154 of transformer 150 is connected to a load 156which can be a quadrupole mass spectrometer for example. If a quadrupolemass spectrometer is used, then the inductance of the air coretransformer 150 can be adjusted to resonate with a capacitance of theanalyzer. The inductance of T1 can be adjusted either mechanically or bychanging the windings ratio of the transformer. Use of an air coretransformer reduces the weight, and makes it feasible to use such adevice. A transformer-coupled output ensures floating output.

[0014] The secondary 154 output is also connected to an RF detector 160which produces a detection signal 162 with a DC level that isproportional to the amplitude of the RF signal 158 produced at thesecondary 154 of the transformer 150. The RF detector can include, forexample, a rectifying diode. The RF detection signal 162 is coupled toone input of an error amplifier 170. The other input of the erroramplifier 170 receives a command 176 indicative of the desired RF level.A serial input command 172 is connected to digital to analog converter174 which is converted to an analog level 176 indicating the desiredlevel. This analog level 176 is coupled to the second input of erroramplifier 170.

[0015] The error amplifier 170 produces an error output 178 indicatingthe difference between the commanded level 172 and the actual level.This difference is coupled through resistors R8 and R5 to the input nodeof the respective sawtooth amplifiers 120 and 122 where it sums with theflip-flop outputs 108, 110. When the error amplifier output 178 is high,it increases the oscillation signal to a higher level, therebyincreasing the drive to the input of the amplifier 120. This effectivelyproduces more conduction from the transistor 130, thereby increasing theamplitude of the RF signal. The increased-amplitude RF signal isreflected by an increase in the output 162 of the RF detector 160, whichhence lowers the error signal 178.

[0016] This control loop provides extremely stable RF and DC voltages.Hence, this system can be used for long term unattended operation in achanging external environment, such as in space or under highly variabletemperatures.

[0017] An important feature of this circuit is its ability to obtain alarge dynamic range output signal. At low levels, the drive signal cancouple through the gate of the MOSFET, and generate an output signalwhich is much greater than the desired minimum signal. In fact, thedesired minimum signal for a quadrupole mass analyzer is about thatnecessary to separate one atomic mass unit. In order to avoid thecoupling-through operation, a cascade stage MOSFET 134 is placed inseries with a diode 136. The MOSFET is biased to bias level VB. Thisprovides the isolation ot avoid the punch through phenomena noted above.

[0018] Another problem is based on the characteristics of operationalamplifiers that are commonly used for this system. Most operationalamplifiers have peak voltages of about 3 to 4 volts for the sawtoothwave produced by the amplifiers. This level might not be high enough tobias the available MOSFETs to drive enough power at the output levels.The peak voltage of the sawtooth is hence increased, by referencing thereturn of the main power supply to a negative voltage at node 131. By sodoing, the peak value seen by the MOSFET is increased by the level ofthe negative voltage present at the return of the driving power source.

[0019] Other embodiments are within the disclosed system.

What is claimed is:
 1. A method of driving a quadrupole massspectrometer, comprising: obtaining an air core transformer with aprimary and a secondary; matching said secondary to the massspectrometer; and driving the primary based on first and second voltagelevels, said driving being via an isolating stage that minimizes lowlevel drive signal coupling.